1. Field of the Invention
This invention relates to a ventilating unit for a carburetor attached to an internal combustion engine. More particularly, to main air paths having their lower ends open to a space above a surface of fuel in a float chamber of the carburetor. A plurality of branches communicate with upper ends of the main air paths via upper ends thereof and are open to the atmosphere via lower ends thereof.
2. Description of Background Art
Japanese Patent Laid-Open No. Hei 7-166961 exemplifies a ventilating unit for a carburetor which includes two branches.
FIG. 6 shows an example of existing ventilating units for a carburetor. In FIG. 6, a carburetor body 1 includes a float chamber 3 provided under the carburetor body 1. A space 3a positioned above a surface of fuel in the float chamber 3 opens to the atmosphere via a pair of ventilating units. Each ventilating unit includes a passage 30 in the shape of letter T. In the passage 30, a portion 30.sub.1 serves as a main air path 12, and extends from an end on the ceiling to a merging point 30a. A portion 30.sub.2 extends laterally from the merging point 30a and then vertically downwardly to the carburetor body, and is coupled to a first ventilating tube 16, thereby forming a first branch 13. A portion 30.sub.3 extends upwardly from the merging point 30a and connects to a second ventilating tube 18 in the shape of an inverted letter U, thereby forming a second branch 14.
Even if a motorcycle including the foregoing ventilating system runs on a rough road, extensive rippling occurs on the surface of the fuel f in the float chamber 3 and fuel f partially intrudes into the main air paths 12 and the first branches 13 and the second branches 14 allow atmospheric pressure to act on the main air paths 12 and the first branches 13. In such a case, the fuel intruding into the main air paths 12 and the first branches 13 is separated, so that a part thereof is returned to the float chamber 3 via the main air paths 12 while the remaining fuel will be quickly dispersed into the atmosphere via the first branches 13. Therefore, it is possible to minimize a period during which fuel stays in the ventilating unit 10 and to maintain the float chamber 3 at atmospheric pressure.
In the related art, each merging point 30a where the main air path 12 and the first and second branches 13 and 14 communicate with one another has a cross-sectional area that is substantially equal to those of the passage 12 and the branches 13 and 14. If a relatively large amount of fuel f gushes into the main air paths 12 from the float chamber 3, it also enters into the first and second branches 13 and 14 at the same time, as shown in FIG. 7. In such a case, it is difficult for atmospheric pressure to act on the merging point 30a, which delays the flow of fuel f to the float chamber 3 and the atmosphere. In this state, the float chamber 3 is isolated from the atmosphere. As a result, a fuel nozzle communicating with the float chamber 3 under the upper surface of fuel f may infect fuel with reduced efficiency, which may adversely affect ease of riding.